Turning to FIGS. 1-3, an example of a PWM amplifier 100 can be seen. In operation, the PWM amplifier 100 receives input signal IN at the noise shaping circuit 102 (which is typically a sigma-delta modulator). Assuming that the noise shaping circuit 102 is a sigma-delta modulator or SDM that uses oversampling, this noise shaping circuit 102 can spread the total noise power over the oversampling frequency band (which is generally larger than the band-of-interest) so as to reduce in-band noise. The noise shaped signal is then applied to the digital PWM 104 so as to generate a PWM signal for use by the amplification stage 106 (which can, for example, be comprised of a digital-to-analog converter (DAC) 108 and amplifier 110 (i.e., class AB) as shown in FIG. 2 or a switching amplifier 112 (i.e., class D) as shown in FIG. 3).
One problem with this amplifier 100 is the nonlinear nature of the digital PWM 104. Some of the in-band nonlinearity associated with the digital PWM 104 can be corrected using predistortion or feedback control, but signal images and nonlinear components can be created at high frequencies (as shown in FIGS. 4A and 4B). As a result of having this high frequency content, the amplification stage 106 should have high linearity; otherwise the high frequency content will fold in-band, limiting in-band linearity. Additionally, this high frequency content can unnecessarily use power. These high frequency content also should be attenuated by high-order analog filters in order to meet spectral requirements. Thus, there is a need for an improved PWM amplifier.
Some examples of conventional systems are: U.S. Pat. No. 7,209,064; U.S. Pat. No. 7,327,296; U.S. Pat. No. 7,425,853; U.S. Pat. No. 7,782,238; and U.S. Pat. No. 7,830,289.